There is a wide industrial demand for alloys capable of long service life in such forms as fixtures, trays, baskets and jigs that are exposed to carburization, oxidation and, possibly, sulfidation, in a variety of heat treating applications. They must often also resist thermal fatigue and thermal shock in applications where quenching is involved. These same properties are desireable in equipment for waste incineration, containers for baking carbon products, retorts and muffles, salt bath fixtures, radiant tubes and burners. Alloy cost is a significant factor, so that very high cost nickel and cobalt base alloys have extremely limited use for this kind of service even though many of them have excellent properties. In such applications required service temperatures are typically in the range of about 1550.degree. to 1700.degree. F. but may extend to as high as 2000.degree. to 2100.degree. F. in some applications.
The wrought form of 310 alloy and its cast equivalent, ACI type HK, have represented the lowest cost material generally suitable for such applications. Nominal composition and 10,000-hour rupture life at several temperatures for this alloy along with a few other alloys are given in Table I.
TABLE I ______________________________________ WEIGHT PERCENT STRESS FOR 10,000-HR NOMINAL RUPTURE LIFE, PSI COMPOSITION 1400.degree. 1600.degree. 1800.degree. ALLOY Fe Ni Cr Si Al F. F. F. ______________________________________ 310(HK) 53 20 25 .5 -- 4000 1200 540 330(HT) 46 35 17 1 -- 4300 1700 630 601 14 60 23 .2 1.4 8000 3500 1400 RA85H 61 14.5 18.5 3.6 1 5200 2300 800 HF20 67 12 19 1 -- 6100 2700 1050 ______________________________________
Alloy 330 and its cast equivalent, ACI type HT, offer some increase in hot strength and have been employed to a great extent in certain types of heat treating equipment.
Alloy 601 is essentially a wrought nickel-base alloy developed for this type of service. It may be seen from Table I that it offers considerable increase in hot strength but, due to its high nickel content, at substantially higher cost.
A recently developed wrought iron-base alloy of excellent properties and low cost has been marketed under the registered U.S. trademark, RA85H. It may be seen from Table I that this alloy offers hot strength somewhere between the iron-base alloys and the nickel-base 601 alloy at much lower critical element content than any of the other alloys.
Both silicon and aluminum serve as very low-cost, partial substitutes for chromium in developing resistance to oxidation and sulfidation. They both substantially enhance carburization resistance but also tend to reduce hot strengths of iron-base alloys significantly. For example, increasing silicon from 0.6% to 2.55% and aluminum from 0% to about 1% in iron-base alloys of about 31% Cr and 15% Ni contents reduces hot strengths at least 30% over the entire temperature range from 1200.degree. to 2000.degree. F. Another example of the effect of silicon and aluminum is seen in the comparison of the hot strength of alloy HF20 to that of alloy RA85H as shown in Table I. The former alloy is essentially the base alloy from which the RA85H was derived by increasing silicon content and adding aluminum but maintaining the same carbon level. Thus, while alloy RA85H has many other excellent properties, it suffers from a reduction in hot strength characteristics as compared to alloy HF-20.
It is well established that for long life at high stress and high temperature iron-base and nickel-base alloys must retain stable austenitic matrix crystal structures, that is, they must not form significant amounts of ferrite or sigma phase during the manufacture of products or in service. Therefore, the balance of proportions of austenite-forming to ferrite-forming elements must be carefully chosen. This balance is enormously complicated not only because the different concerned elements vary in their relative effects upon matrix structure but also because many of them may form certain compounds and therefore be removed in part or entirely from the matrix reaction.
The elements which favor the austenitic matrix structure are nickel, carbon, nitrogen, cobalt and copper. Those which favor ferritic or sigma phase structures include chromium, silicon, aluminum, molybdenum, tungsten, columbium, tantalum, titanium, zirconium and rare earth elements. Oxygen may even enter into the reaction in air melting practice, because certain of the elements may entirely or in part form stable oxides and thus be removed from the matrix.
Nickel does not form nitrides or carbides in any alloys and nickel-base super alloys do not contain any significant quantities of carbon or nitrogen. Rather, they derive their hot strengths principally by the formation and precipitation of compounds known as gamma prime, which are composed of nickel combined with titanium and/or aluminum. Contrariwise, low-nickel, iron-base alloys do not form gamma prime and depend principally upon the formation of carbides and, to a very much lesser extent, nitrides, to develop hot strength.
Aluminum forms nitrides and oxides, but no carbides, and is a powerful ferrite former and hot strength reducer when present in the matrix. Therefore, while aluminum may typically be present in nickel-base super-alloys in amounts of about 1% to 6% as a strengthener, it is generally not employed in iron-base heat resistant alloys although it is sometimes used in tiny fractions of a percent as a deoxidizing element, if high hot strengths are required. Aluminum has been employed in amounts of about 2% to 15% in very low hot strength, ferritic, heat resistant alloys, but these are totally unrelated to the austenitic alloys discussed here.
Silicon forms oxides but no nitrides or carbides. It is a very powerful ferrite former and reduces hot strengths, as noted above. Silicon is most often employed in steel making practice as a deoxidizer in amounts, typically, of about 0.25% to 1%. While silicon has been employed in corrosion resistant alloys in larger amounts, most heat resistant alloy specifications set limits of 1% or 2% or, very infrequently, of 2.5% maximum.
Therefore, alloys of the RA85H type are metallurgically quite different from virtually all other heat resistant alloys, in that they contain large amounts of silicon, and, optionally aluminum. All other ferrite formers that are commonly employed in heat resistant alloys are also strong carbide formers. Many of them also form stable nitrides.
Small additions of two or more special elements have been made in recent decades to various iron-base heat resistant alloys in order to enhance their hot strengths and service lives. For example, U.S. Pat. No. 4,077,801 discloses the addition of combinations of tungsten and titanium or of tungsten, columbium and titanium to several grades of standard heat resistant alloys to improve hot strength. Further enhancement of the properties of the standard heat resistant alloys and similar alloys is described in U.S. Pat. No. 5,077,006 which discloses small additions of the six components molybdenum, tungsten, columbium, titanium, zirconium and one or more rare earth metals. A very important effect of such alloying additions is how they alter the form, solubility, location in the metallic body and tendency to coalesce of the carbides in the altered base alloy. Large total contents of such elements generally reduce elongation, toughness, machinability, weldability and resistance to thermal fatigue and shock. They also increase cost. Therefore, it remains desirable to find the best combination of elements in the lowest contents that will accomplish the desired results in each alloy type for each kind of application.
Also, British Patent No. 1,534,926 discloses high silicon content (4.1% to 12%), corrosion resistant alloys.